Dispensing method

ABSTRACT

In a method for cleaning a liquid delivery device such as a dispensing or pipetting device, the liquid chamber can be flushed in a cleaning step. To remove gas bubbles which might exist in the liquid chamber, for example, the liquid in the liquid chamber is vibrated during the cleaning step.

1. FIELD OF THE INVENTION

The invention relates to a dispensing method for dispensing chemicaland/or biological liquids in minimum amounts by means of a liquiddelivery device, particularly a micropump in a dispensing and/orpipetting device and/or other microsystems.

2. DISCUSSION OF THE BACKGROUND ART

For dispensing and pipetting very small liquid amounts in the range of afew pl to μl, micropumps, for example, are utilized as a liquid deliverydevice, which eject drops through a nozzle of the dispensing orpipetting device. As a micropump, such dispensing or pipetting devicescomprise a liquid chamber that contains a sample liquid or a systemliquid. A wall of the sample chamber is configured to be, e.g., elastic,particularly as a diaphragm. A pulse generator such as a piezo actuatoracts upon the elastic wall. Pressure pulses can be generated in theliquid chamber by the piezo actuator. They cause a delivery of liquiddrops from the nozzle. The droplet volume is clearly defined so that theliquid amount delivered is clearly specified via the number of droplets.The volume of the individual drops is in the range of from 50-100 pl.

In principle, micropumps are very liable to fault. This is due to thevery small dimensions and the sensitivity to precipitations and gaggingsrelated thereto. Because of its compressibility, the presence of an airbubble, e.g., may result in that no delivery effect or a too smalldelivery effect occurs even if the diaphragm of the micropump isactuated. Furthermore, surface tension effects may impair the perfectfunction of such micropumps. Particularly with automation such as, forexample, in medium or high throughput screening, a high reliability ofsuch micropumps is required. Malfunctions which might nevertheless occurshould be detected and repaired automatically. One problem which oftenoccurs with micropumps, e.g., is that precipitations, e.g. ofcrystallizations of the liquid in the liquid chamber, are producedwithin the liquid chamber. Further, gas bubbles, particularly airbubbles, are often produced within the liquid chamber, a channelconnected with the liquid chamber or the nozzle. This often results in afailure of the micropump. With micropumps used in dispensing andpipetting devices, gas bubbles at least lead to a falsification of thedelivered drop size and thus to a falsification of the delivered liquidamount if they will not directly result in the failure of the micropump.

To avoid malfunctions in such liquid delivery devices, it is known toflush the liquid chamber with liquid in a cleaning step. The liquid inthe liquid chamber, i.e., the sample or system liquid, is used forflushing. It is also possible to flush the chamber with flushing liquidvia a separate connection of the liquid chamber with a reservoir. Ifnecessary, the pump may be operated at an increased frequency oramplitude. Such known flushing methods, however, are time-consuming.This is particularly disadvantageous if such liquid delivery devices areused in medium or high throughput screening. Further, the liquidconsumption in known flushing methods is extremely high. This isparticularly disadvantageous when the flushing in a dispensing devicehas to be done with expensive sample liquid.

It is the object of the invention to provide a method for cleaning aliquid delivery device by means of which a quick and reliable cleaningis guaranteed.

SUMMARY OF THE INVENTION

In the method for dispensing chemical and/or biological liquidsaccording to the invention, liquid is delivered in a dispensing step bymeans of a dispensing device. To this end, a pulse generator such as apiezo actuator acts upon a liquid chamber which contains the sampleliquid to be delivered. The pulse generator generates a pressure pulsein the liquid chamber so that a liquid droplet is delivered through acapillary channel. During a dispensing step, several droplets,preferably at least 5, at least 10 in a particularly preferred manner,and at least 20 droplets in particular are delivered. Particularly, upto 500, if necessary, even up to 1,000 droplets, can be delivered in adispensing step. The number of the droplets delivered corresponds withthe number of pulses of the pulse generator so that the delivered liquidamount can be directly defined since the volume of the individualdroplet is known.

Before or after such a dispensing step, a cleaning step is performedaccording to the invention, wherein flushing liquid is passed throughthe liquid chamber. Sample liquid can also be used as a flushing liquid.

According to the invention, the medium in the liquid chamber of theliquid delivery device, i.e., particularly the liquid and existingimpurities, are vibrated during the cleaning step in which the liquidchamber is flushed. By vibrating the medium, it is possible to act uponimpurities, particularly gas bubbles, existing in the liquid chamber insuch a manner that they disintegrate. Impurities which may be bothparticles and gas bubbles disintegrate into smaller parts and smallerbubbles, respectively. These can be flushed out of the liquid chambermore easily. Thereby, for example, a gagging of the nozzle because ofrelatively large impurities is avoided if the liquid delivery device isa dispensing or pipetting device. By vibrating the medium located in theliquid chamber and the disintegration of the impurities, particularlythe gas bubbles, caused thereby, they can be flushed out of the liquidchamber more easily. The method according to the invention isparticularly suitable for flushing out gas bubbles which cannot beflushed out or only difficulty so by means of known flushing methodssince gas bubbles adhere to the surfaces of the dispensing device. Thus,the liquid chamber can be cleaned faster. This is a particular greatadvantage with automatic methods such as the medium or high throughputscreening. Thus, it is possible by means of the method according to theinvention to considerably reduce the cleaning times. Further, thereliability is increased by the method according to the invention sincelarger impurities disintegrate and c an thus be reliably removed fromthe liquid chamber. Furthermore, the consumption of liquid in a cleaningstep is considerably reduced. This is particularly advantageous withdispensing devices in which the cleaning is effected by means of sampleliquid.

In a particularly preferred embodiment of the invention, the frequencyof the vibrations occurring in the liquid is varied during a cleaningstep. To this end, the frequency of a pulse generator acting upon theliquid is varied, the variation of the frequency of the pulse generatorprovoking a variation of the vibration frequency of the medium. It isparticularly preferred if a piezo actuator or a corresponding componentis used as a pulse generator to this end. If the liquid delivery deviceis a dispensing or pipetting device, it is particularly preferred to usethe pulse generator such as, for example, the piezo actuator, by whichthe drop delivery is caused, as a pulse generator for generating thevibrations in the liquid as well. This has the advantage that noadditional component is required to perform the cleaning step includingthe flushing of the liquid chamber and the vibrating of the mediumaccording to the invention.

Varying the vibrational frequency has the advantage that impurities orgas bubbles of different sizes can be resonated whereby thedisintegration process is triggered. Thus, varying the frequency resultsin a further reduction of the flushing times. This increases thereliability of the liquid delivery device and reduces the liquid amountrequired for flushing. This is particularly advantageous with medium andhigh throughput screening. The frequency or the different frequencieswhich are produced during a cleaning step can be advantageouslyselected, for example, in dependence on the used liquid and the dangerof larger or smaller agglomerates resulting therefrom, for example. Thefrequencies can also be matched to the fact that a liquid has a strongeror weaker tendency to develop gas and thus forms gas bubbles.

Experiments have shown that a minimum frequency of at least 1 kHz,preferably at least 3 kHz, is particularly advantageous when gas bubblesoccur. Very good results can further be achieved when the maximumfrequency amounts to 60 kHz at maximum, preferably 40 kHz at maximum.Within the minimum frequency and the maximum frequency, the frequency ischanged continuously or stepwise. Good cleaning results can be achievedwhen the frequency is increased stepwise by 200-250 Hz per step.Starting from a minimum frequency, the frequency is preferably increasedtoward the maximum frequency. Since larger bubbles disintegrate intosmaller and smaller bubbles and particles disintegrate into smaller andsmaller particles, respectively, it is advantageous to increase thefrequency to a maximum frequency starting from the minimum frequency forthe disintegration into bubbles or particles which are as small aspossible and adapted to be flushed out can be accelerated. If necessary,changes between individual frequencies are possible so that thefrequencies are repeatedly increased and decreased during one cleaningstep. Furthermore, a combination of both processes is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, the invention is explained in detail with respect to apreferred embodiment with reference to the accompanying drawings.

FIG. 1 is a schematic front view of a liquid delivery device,

FIG. 2 is a schematic sectional view along the line II-II in FIG. 1,

FIG. 3 is an example of an excitation signal for exciting piezoactuators, and

FIG. 4 shows a frequency course according to the invention during acleaning step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The liquid delivery device 10 illustrated in FIGS. 1 and 2 is, e.g., adispensing device. It comprises a liquid chamber 12 connected with areservoir via a channel 14. In a dispensing device, the reservoircontains the sample liquid dispensed in the form of drops 18 through anozzle or the capillary channel 16. If necessary, the liquid chamber 12is further connected with another reservoir containing flushing liquidfor performing a cleaning step. A piezo actuator 20 acts upon the liquidin the liquid chamber 12. The piezo actuator 20 acts upon an elasticwall 22 of the liquid chamber 12. By applying a voltage to the piezoactuator 20, the elastic wall 22 is deformed and a pressure pulse isexerted upon the liquid in the liquid chamber 12. Thereby, a drop 18 isdelivered, e.g., toward a microtiter plate 24 for filling wells 26located in the microtiter plate 24.

The excitation pulse form shown in FIG. 3 is a possible voltage pulsesequence applied to the piezo actuator 20 during the dispensing step forproducing droplets 18.

Over a time period t₁, the square pulse illustrated in FIG. 3 produces avoltage applied to the piezo actuator 20, which is repeated after aperiod T. By each excitation pulse 28, a droplet 18 is delivered.Instead of a square pulse, a trapezoidal pulse, for example, or a squarepulse with an obliquely falling edge, for example, can be used as well.It is also possible to apply polygonal pulses to the piezo actuator 20,a voltage pointing to the opposite direction being applied before orafter the excitation pulse 28, if necessary. Further, it is alsopossible, for example, to apply sinusoidal excitation pulses to thepiezo actuator 20 or another pulse generator.

During a cleaning step R (FIG. 4) performed before or after thedispensing step, an excitation frequency that is, e.g., sinusoidal, isapplied to the piezo actuator 20. The frequency may be continuouslyincreased, e.g., from a minimum frequency f_(min) at a time t₁ up to amaximum frequency f_(max) at a time t₂. After the time t₂, the frequencyis then reduced continuously down to a minimum frequency t_(min) until atime t₃. During such a continuous or perhaps stepwise variation of thefrequency, preferably more than 50, preferably more than 80 andparticularly preferably more than 120 vibration periods are produced.This is preferably effected per frequency step.

A cleaning step according to the invention can be particularly performedafter a fault has been detected. If necessary, it is also additionallypossible to preventively perform a cleaning step, at predeterminedintervals, for example, or after defined operational situations. It ispossible, but not required, to perform a cleaning step R before or aftereach dispensing step.

1. A dispensing method for dispensing chemical and/or biological liquidsin minimum amounts, wherein in a dispensing step, several droplets aredelivered by a dispenser by a pulse generator acting upon a liquidchamber to deliver droplets through a capillary channel, and, in acleaning step, flushing liquid is passed through the liquid chamber,wherein, during the cleaning step, the medium in the liquid chamber isvibrated in order to destroy impurities.
 2. The method according toclaim 1, wherein the frequency of the vibrations is varied during saidcleaning step.
 3. The method according to claim 1, wherein thevibrations are generated by a pulse generator acting upon an elasticwall of the liquid chamber.
 4. The method according to claim 3, whereinthe frequency of the pulse generator is varied during said cleaningstep.
 5. The method according to claim 1, wherein the frequency isselected such that impurities disintegrate.
 6. The method according toclaim 1, wherein a minimum frequency (f_(min)) during said cleaning stepamounts to at least 1 kHz.
 7. The method according to claim 1, wherein amaximum frequency (f_(max)) during said cleaning step amounts to 60 kHzat maximum.
 8. The method according to claim 1, wherein the frequency isincreased stepwise from a minimum frequency (f_(min)) and/or isdecreased stepwise from a maximum frequency (f_(max)).
 9. The methodaccording to claim 1, wherein, during the dispensing step, the pulsegenerator is operated with an excitation pulse serving to deliverdroplets.